The present invention relates to a cord-actuated drum, which drives a mechanical system. The preferred embodiment was designed for use in a cord tilter used for opening and closing coverings for architectural openings such as Venetian blinds, pleated shades, and other blinds and shades. Typically, a blind transport system will have a top head rail, which both supports the blind and hides the mechanisms used to raise and lower or open and close the blind. Such a blind system is described in U.S. Pat. No. 2,614,623, xe2x80x9cNelsonxe2x80x9d, which is hereby incorporated by reference. The raising and lowering is done by a lift cord attached to the bottom rail (or bottom slat). The tilting of the slats to open and close the blind is typically accomplished with ladder tapes (and/or tilt cables). The lift cords (in contrast to the tilt cables) typically run through holes in the middle of the slats and are connected to the bottom rail.
A prior art cord tilter is shown in Canadian patent application 2,206,932, which is hereby incorporated by reference, and in FIGS. 2 and 3 of this application. Cord tilters for window covering products typically have one thing in common, and that is that a cord wraps around a driving drum with opposite ends of the cord extending vertically downwardly from opposite sides of the drum. The ends of the cord may be two loose ends or they may be a closed loop. Most cord drums are wrapped from one end of the drum to the other end of the drum with a first cord end such that when this first cord end is pulled to unwind, a second opposite cord end is wrapped onto the cord drum. The first and second cord ends may be on a single cord, or they may be ends of two separate cords. Also, the cord may be a continuous loop, so that the two cord ends are not technically ends at all. Typically, there is a clearance of less than two cord diameters between the drum and the housing surrounding the drum, such that only one cord diameter can fit between the drum and the housing, and the housing urges the cord to track properly, preventing over-wraps which would cause the cord to tangle and bind against the housing. However, over-wraps may still occur for a number of reasons. For example, when the cord is in tension, the diameter of the cord actually may reduce from its diameter in a normal condition, so two cords may be able to pass each other even if the clearance between the housing and the drum is less than two normal cord diameters. Also, the cord wraps best onto the drum when it enters the drum at right angles to the axis of rotation. As the angle of entry moves away from right angles and approaches a direction that is more parallel to the axis of rotation, the likelihood for an overwrap condition increases.
If the user tends to hold on to both cord ends at the same time (creating a back pressure on the cord end that is being wound up while pulling downwardly on the cord end that is being unwound), the likelihood for over-wrap increases. When pulling on both cord ends simultaneously, a much greater force is exerted on the operating system than when pulling on a single cord end, because one must overcome not only the required system inertia to cause the tilting action, but one must also overcome the opposing force being placed on the upward-moving cord end.
This extra force puts greater tension on the cords, which tends to reduce the diameter of the cord, so that two cord portions may be able to cross over in a space, even if the clearance is less than two normal cord diameters.
This extra force also causes a much greater deflection in the componentry than would otherwise be present, thus causing a larger clearance to occur between the housing and the top of the driving drum, which then increases the likelihood that an over-wrap condition will occur. At the same time, the clearance between the housing and the bottom of the drum is reduced, which may lead to pinching and binding of the cord between the housing and the drum.
In addition, the backward pressure on the upward moving cord end inhibits that cord end from following its natural circuitous path in its upward or winding process.
A primary objective of the present invention is to provide an improved cord drive mechanism which addresses and solves the drawbacks of the mechanisms found in the prior art.
The preferred embodiments shown herein change the exit point and direction of the two cord ends as they exit the driving drum in order to improve the angle at which the cord ends enter into contact with the drum and in order to have the two cord ends apply force to the drum in opposite directions, so that, if both cord ends are pulled at the same time, the forces cancel each other out rather than adding together to cause deflection of the drum relative to its housing.
Furthermore, because deflection of the drum relative to the housing is greatly reduced or eliminated, the clearances between the drum and the housing can be reduced, by design, to the level of 1.1 to 1.6 cord diameters, thus encouraging the cord to be initially laid down on the drum in the proper position without any over-wrap, and maintaining cord placement on the drum even in conditions where the cord may become completely relaxed.
While the embodiments of the present invention described below show a typical horizontal Venetian blind, it should be obvious to those skilled in the art that a cord-driven drum made in accordance with the present invention may be used in a wide variety of different arrangements in which a mechanical drive is required, and the orientation of the drum may be in any direction.